1. Field of the Invention
This invention relates to aircraft collision avoidance systems, and more particularly, aircraft proximity warning systems utilizing both RF signals and optical radiation pulses.
2. Description of the Prior Art
In the prior art, an aircraft proximity warning indicator (PWI) on a protected aircraft utilized either a radio or optical signal from an intruder aircraft to provide an indication of its presence. One example of a radio signal system is where the protected aircraft interrogates transponders of intruder aircraft. A decodable reply from all intruder aircraft equipped with transponders provided the protected aircraft with the range of each intruder aircraft using the round trip signal transit time. The decodable reply may also contain other desirable information such as the barometric altitude of the intruder aircraft. In implementing an RF transponder system, relatively expensive equipment is required in all aircraft. Private aircraft, which present a significant collision threat to other aircraft in the air, for example commercial aircraft, may not be able to utilize the RF transponder systems because of the expense of the equipment.
A second relatively low cost approach for proximity warning is to detect optical signals from a standard xenon flashing beacon installed on an intruder aircraft by an electro-optical receiver in an aircraft to be protected. The use of such optical beacons is now widespread on private aircraft and could be used by all aircraft without serious economic hardship. Experiments have shown that optical beacons could be detected at satisfactory ranges of up to 10 miles by a simple detector and optics system. A standard aircraft flashing beacon flashes at a rate of about 30 per minute for a duration of 50 microseconds. A conventional receiver could provide detection of the optical beacon over a field of view of 360.degree. azimuth and .+-. 45.degree. elevation with angular resolution of 1.degree. but would have to be a "staring" system rather than a gated or scanning system. In a "staring" system, the receiver is sensing optical radiation 100% of the time. A severe disadvantage of a "staring" system is that light from other sources is allowed to accumulate in the sensor between flashes of light from an optical beacon. The light from other sources may be for example streetlights, daylight, or reflections of sunlight. The light from other sources will appear as background noise in a "staring" system with the optical beacon flash being the desired signal. The optical receiver in a "staring" system will therefore have to operate with a lower signal to noise ratio since the background noise is accumulated 100% of the time. The present invention, by use of synchronization signal prior to an optical radiation pulse allows the sensor to activate during an optical radiation pulse and off at other times thereby providing a higher signal to noise ratio in the optical receiver.